Effects Of Stand Density On Soil Respiration In Larch Plantations

Density determines the spatial structure of stand and has important effects on carbon cycle of forest ecosystems.In order to understand the effects of stand density on soil respiration of Larch(Larix sp.)plantations,10-year-old sapling plantations with different planting densities(2000 trees·hm-2 3300 trees·hm-2 and 4400 trees·hm-2)were selected in Maoershan mountain area of Heilongjiang province,to measure soil respiration rate and analyze its factors in the growing seasons in 2016 and 2017.Results show that:(1)The effects of stand density on the mean soil respiration rates were different in two growing seasons.In 2016,mean soil respiration rates decreased with the increase of stand density,and the difference between low density stands and high density stands was significant(P<0.05).In 2017,mean soil respiration rates of low density stands and middle density stand were similar,and those of high density forest was lower.There were no significant differences among three stands(P>0.05).Soil respiration rates of three stands showed obvious seasonal variation during growing seasons,the highest in June and July while the lowest in the end of October.At different times,differences of soil respiration rates in three density stands was different.The soil respiration rates of Larch sapling plantations ranged from 0.30 to 5.15 ?mol CO2·m-2·s-1.Q10 of different stands were different,Q10 of middle density stands is the largest and that of low density stand is the second,Q10 of high density stand is the lowest.(2)Forest temperature,soil temperature and soil moisture decreased with the increase of stand density.Contents of soil total carbon and nitrogen in low density stands were lower than those in the other two stands.There were no significant differences among total phosphorus contents of three density stands(P>0.05).C/N,C/P and N/P of low density stand were significantly lower than those of medium density and high density stands.Soil ammonium nitrogen contents decreased with the seasonal change and then increased.Soil nitrate nitrogen contents gradually increased with the seasonal change,and the ammonium nitrogen and nitrate nitrogen had no obvious changes in the different density stands.Contents of available phosphorus in soil increased first and then decreased with seasonal changes,and increased with stand density.(3)Annual yield of litter in Larch sapling stands increased with the increase of stand density,but the difference were not significant(P>0.05),and the amount of ground litter in medium density stands and high density stands were significantly higher than that of low density stands(P<0.05).Stands with low density has the highest rates of litter decomposition while stands with middle density has the lowest.The biomass of fine roots decreased with the increase of stand density,and there was significant difference between high density stands and other density stands(P<0.05).The biomass of fine roots showed a certain seasonal variation.There was no significant difference in soil microbial biomass C among three forest stands(P>0.05).(4)There were positive correlations between soil respiration rates and soil temperature,soil temperature,soil moisture and fine root biomass,and negative correlation with soil ammonium nitrogen contents and 0?10 cm soil nitrate nitrogen contents.Soil respiration rates were not related to soil available phosphorus contents and soil microbiological biomass carbon contents.